Telephone system



April 26, 1949. co -1 1- 2,468,040

TELEPHONE SYSTEM Filed April 13, 1943 E 2 Sheets-Sheet 1 INVENTOR 54/4 cal /1P7 April 26, 1949. E col-[RT 2,468,040

TELEPHONE SYSTEM Filed April 13-, 1943 '2 Sheets-Sheet 2 INVENTOR [/G/L (UH/P7- t. E A'ITORNEY Patented Apr. 26, 1949 TELEPHONE SYSTEM Eigil Cohrt, Copenhagen, Denmark, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application April 13, 1943, Serial No. 482,901 In Denmark November 23, 1939 Section 1, Public Law 690, August 8, 1946 Patent expires November 23, 1959 11 Claims.

The present invention relates to automatic private branch exchange systems, and, more particularly, to systems of this type including arrangements whereby a subscriber, who is engaged in an outside call over a junction to the main exchange, may hold this junction while he calls another subscriber at the private branch exchange and, if desired, may transfer the connection with the junction to the other subscriber.

In known exchanges of this type, in which all the junctions terminate in a combined line finder and final selector assembly, the call-back circuits connect each junction to a set of blank contacts on each local line finder. This means that the capacity of the local line finder assembly is decreased by one subscribers line per junction so that in a private branch exchange with a capacity of 200 lines on its local line finder assembly and with 20 junctions, only 180 subscribers may be connected.

One object of the present invention is to provide, in a private branch exchange system of the type described, call-back arrangements between the junctions and the local line finder assembly whereby the number of bank contacts in the local line finders occupied by the call-back circuits is considerably smaller than the number of junctions employed in the system.

Another object of the invention is to provide means whereby, upon transmission of a callback signal over a junction, one of a limited number of call-back circuits is selected to connect the calling junction to a free local line finder.

A still further object of the invention is to provide means whereby all the free call-back circuits are incapacitated whenever all the local line finders and their associated link circuits are occupied.

These and other objects are accomplished, according to the invention, by the arrangement and combination of elements set forth in the following description, deflned in the appended claims and illustratively exemplified in the ac companying drawing, in which:

Fig. 1 is a simplified circuit diagram of an automatic private branch exchange system with a call-back arrangement according to one embodiment of the invention;

Fig. 2 is a more detailed wiring diagram of a call-back circuit with portions of a junction and a local line finder as used in the system according to Fig. 1,

Fig. 3 is a simplified circuit diagram of an automatic private branch exchange system with 2 a call-back arrangement according to a second embodiment of the invention; and

Fig. 4 is a more detailed wiring diagram of a call-back circuit with portions of a junction and a local line finder, as used in the system according to Fig. 3.

Referring now to the figures, and first to Fig. 1, SB is a subscribers station which is connected to a junction TCT over a combined final selector and line finder assembly LF/FS and, from said junction TCT over one of a relatively small number of call-back circuits RTC to a local line finder assembly LF. Each local line finder LF is connected by a link circuit LCT to a. local final selector FS.

Each call-back circuit RCT comprises a callback finder P and three control relays Ltr, Shr, Lhr' (see Fig. 2). Furthermore, a single startin relay Fsr is connected in parallel to all the junctions TCT and controls at its front contact a starting relay Asr common to all the call-back circuits RCT.

The system operates as follows: If a subscriber, who is engaged with a call over a junction, wishes to make a call-back or to transfer the connection, he operates a mechanism (not shown) to energize a call-back relay Arr in the junction connected to his subscribers station SE. The relay A231, at its front contact, connects battery over a resistance R3 to energize relay F81 and starting relay Asr. The latter, at its front contacts, connects, in each free call-back circuit RCT, ground over a back contact of a relay Br common to all the call-back circuits RTC and over back contacts of the individual relays Ltr and Lhr to a driving magnet PM of the corresponding call-back finder P, whereupon the latter starts to rotate and to hunt for the calling junction TCT. When the brush t of the call-back finder P reaches the bank contact corresponding to the calling junction TCT which bank contact is now connected to the test battery T through the front contact of the callback relay Am and the resistance R2, the arrester relay Ltr is energized and opens at its back contact the energizing circuit for the driving magnet PM so that the call-back finder P stops in this position With its brushes a-d on the bank contacts corresponding to that occupied by the brush t. Simultaneously, an energizing circuit for the selector relay S121 is closed at the front contact of the relay Ltr. Selector relay Shr, at its outer right hand front contact closes a stick circuit for itself shunting the front contact of the starting relay Asr, and, at its inner right hand front contact, it introduces the low resistance finders LP. The brushes of a local line finder forming part of a free link circuit LCT come to rest on the set of bank contacts containing the contact connected to the test battery T, and a circuit is closed from ground over the brush of the line finder LF, winding of the interrupter re lay Lhr in the call-back circuit RCT and a restolf ing relay Cor in the calling junction TCT to battery. The junction TCT is now metallically connected through to the selected link circuit LCT over the brushes a, b of the selected call-back finder P and the brushes a, b of the selected local finder LF so that the subscriber can make the local-call and, if desired, transfer the connection.

The energization of the restoring relay Cor causes the relays Fsr, Asr, Li? and SM to release, andthe interrupter relay Lhr disables, at its right hand back contact, the energizing circuit for the driving magnet PM so that an occupied call-back circuit is not aiiected by a subsequent call-back signal from another junction.

Each link circuit LCT has associated therewith a contact c1 which is closed when the link circuit is taken into use. The contacts er of all the link circuits LCT are connected in series in the energizing circuit of a disabling relay Br. Thus, when all the link circuits are occupied, relay Br is en' ergized and at its back contacts breaks the energizi'ng circuit of all the magnets PM thus dis-- abling all the call-back circuits RCT which are not occupied.

In the arrangement described the number of call-back circuits and the number of bank contacts of the local line finders occupied by the callback circuit connections can be considerably smaller than the number of junctions in the systern, because generally only a few call-backconversations take place simultaneously and a transfer takes only a few seconds. Thus a considerable saving is accomplished. For instance, in a private branch exchange system having a local line finder assembly with 200 contact places and 20 junctions, four call-back circuits ofthe type described will generally suffice to meet all requirements, and consequently only four contact sets in each local line finder are occupied by the call-back connections. Where power drivenline finders with 102 contact sets are used the callback circuits may be connected to the test positions. If other line finders are employed, only four of the" positions corresponding to subscriber ines are needed for the call-back circuits, so that the exchange will have a capacity of 196' subscriber lines instead of 180 in previous arrangements.

The modified system according to Fig. 3 is similar to that of Fig. 2, with the exception that, according to Fig. 3, a smaller step switch S is associated with each junction TCTa, each switch S having, for instance, ten contacts in its contact bank. Each individual call-back circuit RCTa comprises two control relays Sir and Lhm, and each junction TCTa includes, in addition to the step switch S, a starting call-back relay Arm, a stepping magnet SM, and an arrester' relay 4 Ltra (see Fig. 4). The input leads of each ca lback circuit RCTa are connected to a predetermined set of bank contacts in each step switch S5; The system operates as follows:

In ca'seof a call for a second'localparty or of a transfer, the call-back relay Astra in the calling junction TCTc is energized and closes an energizing circuit for the magnet SM of the stepping switch S over a back contact of the arrester relay can while simultaneously connecting ground through the high resistance Winding of relay Lira to the t brush of the associated switch S. The switch'S starts to rotate and hunts with the brush t for a free call-back circuit which is characterized by battery through the resistance R5 and the winding of the selector relay Sir. When a free call-back circuit RTCa has been found, the relay Lira; is energized and arrests the step switch S. Simultaneously, the selector relay Sir is energized and, at its right hand front contact, connects ground to the sta-rt circuit SRL of all the local line finders Furthermore, at the left hand front contact of the selector rel'ay Sir, the test battery T is connected through the resistance Rid-t0 the corresponding: bank contacts of the local line finders LFL The brushes ad andt of a local line finder LF associated with a free link circuit LCT stop on their bank contacts'connected with the selected call-back circuit RTC'a, and ground connected to the c brushof this line finder LF operates the interrupter relay I-J'Lra and the restoring relay C01. The relay Cor opens the circuit for the arrested relay Ltra" and t'h'e selector relay St'r, which circuit is further disabled at the right" hand back contact of the interrupter relay Lh'r'a' so that the occupied-call-back circuit RCTc is rendered non-responsive to subsequent calliba'ck' signals from any otherfjunction.

The calling ji'inction TCI'a is now metallicall'y connected through to the selected link circuit LCT over the selected call-back circuit RCTa; If all the link circuits LCT are busy, the disabling" relay Br is energized in the manner described above and disables all the unoccupied call back circuits RCTa by breaking thecircuit through theisel'ector relay Sir.

I claim:

1. In an automatic private branch exchange system" having a number of subscriber stations; a number of link circuits each including a local line finder and a local final selector for establishinglintra-exchan'ge' connections between the vari-' ous subscriber stations, and a limited number of junctionsfor connecting the subscriber stations to an outside net, the combination" of a call-back arrangement whereby a. subscriber; who is engagedin an outside call over a junction, may keep this junction and, at the same time, make a callback to another subscribers station at the private branch exchange, said call-back arrangement comprising a number of call-back circuits less. than the number of junctions in said system, each of said call-back,circuitshaving a plurality of leads connected t'oa predetermined Set of bank contacts in each of said local line finders, means including'a call-back relay in each junction and a plurality of call-aback finders each controlled by at least one of said' call-back relays to connect the callin junction selectively with a free callback circuit, an arresterirelay' for each call-back fil'lde'r'to stop the latter when the connection between the calling" junction and the selected callback circuit has been established,. a common starting circuit for said local line finders, anormany o en test circuit in each call-back circuit,

each individual test circuit including one of said leads connected to the corresponding bank contacts of all the local line finders, a selector relay in each call-back circuit adapted to be energizedupon establishment of a connection between said call-back circuit and the calling junction to close said common starting circuit and the test circuit individual to the selected call-back circuit, whereby the brushes of a free local line finder are arrested on their bank contacts connected with the leads from the selected call-back circuit, and an interrupter relay in each call-back circuit operative when one of said link circuits is connected through said call-back circuit to a calling junction to render said occupied call-back circuit non-responsive to a call-back from another junction.

2. A system, as claimed in claim 1, including a restoring relay in each juction operative when a link circuit is connected through one of said recall circuits to said junction to deenergize the associated arrestor relay and the selector relay in the selected call-back circuit.

3 A system, as claimed in claim 1, including a disabling relay, capable, upon being energized, of rendering all the free call-back circuits nonresponsive to call-backs from the junctions, a normally open energizing circuit for said disabling relay, and means to close said energizing circuit when all said link circuits are busy.

4. A system, as claimed in claim 1, in which each call-back finder is associated with one of said recall circuits and has a normally open energizing circuit forming part of the latter and means are provided to clos all said energizing circuits upon energization of any call-back relay, the circuits of each junction being connected to a predetermined set of bank contacts in each of said call-back finders, and the interrupter relay in each call-back circuit being adapted to disable the energizing circuit of the associated call-back finder.

5. A system, as claimed in claim 1, in which each call-back finder is associated with one of said junctions and each call-back circuit has a number of leads connected to a predetermined set of bank contacts in each of said call-back finders, the interrupter relay in each call-back 6 circuit being adapted to disable the associated selector relay.

6. In an automatic telephone exchange system, a P. B. X. and a main exchange, a plurality of scribers stations connected with the P. B. X., a plurality of junctions connecting the P. B. X. with the main exchange, means including automatic switches for establishing connections between a calling and a called P. B. X. subscriber, a plurality of call-back circuits at the P. B. X less in number than said junctions, switching means controlled by a P. B. X. subscriber, engaged in an outside connection over a junction, for connecting an idle call-back circuit with the junction in use and with said automatic switches through which another P. B. X. station may be called.

'7. The telephone system according to claim 6, and in which the automatic switches comprise pairs of line finder and final selector switches.

8. The telephone system according to claim 6, and in which the switching means comprises a plurality of finder switches, each having a set of wipers in which a call-back circuit terminates and a bank of terminals in which the junctions terminate.

9. The telephone system according to claim 6, and in which the switching means comprises a plurality of finder switches, each having a set of wipers in which a junction terminates and a bank of terminals in which the call-back circuits terminate.

10. The telephone system according to claim 6, and in which the automatic switches comprise line finders having terminals in which said callback circuits and the lines of the P. B. X. subscribers terminate.

11. The telephone system according to claim 6, and means for preventing access to any call-back circuits when all said automatic switches are busy.

EIGIL COI-IRT.

REFERENCES CITED UNITED STATES PATENTS Name Date Richardson May 10, 1927 Number 

